1. Field of the Invention
This invention relates to a thermoplastic elastomer useful for obtaining a molded or otherwise formed product which is excellent in not only flexibility, rubber elasticity and mechanical strength but also in external appearance and touch and is substantially free from bleeding and fogging; and a production process of the thermoplastic elastomer; such a molded or otherwise formed product; and a composition for the thermoplastic elastomer (hereinafter called "thermoplastic elastomer composition").
2. Description of the Related Art
It is known for many years that a syndiotactic polypropylene can be obtained by conducting polymerization at a low temperature in the presence a catalyst composed of a vanadium compound, an ether and an organoaluminum. The polymer available by this process however has low syndiotacticity so that it can hardly be considered to literally have syndiotactic properties. J. A. Ewen et al. found for the first time that polypropylene having such high tacticity as exceeding 0.7 in terms of syndiotactic pentad fraction can be obtained using a catalyst composed of aluminoxane and a transition metal compound having an asymmetric ligand (J. Amer. Chem. Soc., 110, 6255-6, 1988).
The polymer obtained according to the process of J. A. Ewen et al. had high syndiotacticity and was equipped with more elastic properties than isotactic polypropylene. Its flexibility, rubber elasticity and mechanical strength, however, were not sufficient when one tried to apply it as a soft molding or forming material to fields where soft PVC, vulcanized rubber or the like was employed.
It has been attempted to improve the flexibility and impact resistance of isotactic polypropylene, usually by blending an ethylene-propylene copolymer rubber or the like. The resin compositions thus obtained were excellent in flexibility and impact resistance, but insufficient in rubber elasticity, mechanical strength and external appearance.
With a view toward overcoming the above problems, some processes have been proposed, including a process in which a thermoplastic resin composition is dynamically crosslinked with an organic peroxide [as disclosed in Japanese Patent Publication No. 34210/1978 (corresponding to U.S. Pat. No. 3806558)] and a process in which a thermoplastic resin containing a polymer having unsaturated carbon-carbon bonds such as rubber is dynamically crosslinked with a sulfur-base crosslinking agent [as disclosed in Japanese Patent Publication No. 18448/1970 (corresponding to U.S. Pat. No. 4130535)].
These processes are however accompanied by the drawback that when a soft composition containing particularly a large amount of rubber is treated according to any one of these processes, the flowability of the composition is substantially lowered, resulting not only in deteriorated moldability or formability but also in a molded or otherwise formed product having a rough surface and hence poor external appearance and touch.